U.S. patent number 5,164,339 [Application Number 07/707,931] was granted by the patent office on 1992-11-17 for fabrication of oxynitride frontside microstructures.
This patent grant is currently assigned to Siemens-Bendix Automotive Electronics L.P.. Invention is credited to George E. Gimpelson.
United States Patent |
5,164,339 |
Gimpelson |
November 17, 1992 |
Fabrication of oxynitride frontside microstructures
Abstract
Method for producing a low stress silicon oxynitride
microstructure on a semiconductor substrate at temperatures not
higher than 500.degree. C. The method is particularly adapted for
forming integrated silicon sensors where the oxynitride
microstructure is fabricated on a substrate under conditions which
do not harm the integrated circuit electronics.
Inventors: |
Gimpelson; George E. (Newport
News, VA) |
Assignee: |
Siemens-Bendix Automotive
Electronics L.P. (Auburn Hills, MI)
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Family
ID: |
26942678 |
Appl.
No.: |
07/707,931 |
Filed: |
May 28, 1991 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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252801 |
Sep 30, 1988 |
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Current U.S.
Class: |
438/52;
148/DIG.159; 438/619; 438/703 |
Current CPC
Class: |
G01L
9/0042 (20130101); Y10S 148/159 (20130101) |
Current International
Class: |
G01L
9/00 (20060101); H01L 021/30 () |
Field of
Search: |
;437/226,228,194,238,921,235,944,901,966
;73/715,719,720,721,725,726,727,754,34C ;357/26
;148/DIG.159,33.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3234907 |
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0106146 |
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0162448 |
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0025245 |
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0085532 |
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0020352 |
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Jan 1986 |
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0027659 |
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Feb 1986 |
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JP |
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0135141 |
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Jun 1986 |
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JP |
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0137343 |
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Jun 1986 |
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JP |
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0140149 |
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Jun 1986 |
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JP |
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0203654 |
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Sep 1986 |
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JP |
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0206242 |
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Sep 1986 |
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JP |
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0228655 |
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Oct 1986 |
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JP |
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Other References
Wolf et al., Silicon Processing for the VLSI Era, vol. 1-Process
Technology, Lattice Press, 1986, pp. 535-536. .
Lee et al, "Silicon Micromachining Technology . . . " SAE Technical
Papers, pp. 1-10. .
Allan, Roger, "Sensors in Silicon", High Technology/Sep. 1984, pp.
43-77..
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Primary Examiner: Wilczewski; Mary
Attorney, Agent or Firm: Boller; George L. Wells; Russel
C.
Parent Case Text
This is a continuation of application Ser. No. 07/252,801, filed
Sep. 30, 1988, now abandoned.
Claims
I claim:
1. A method for producing a low stress free-standing
microstructure, said method comprising the steps of:
providing a silicon substrate;
forming a spacer layer on said silicon substrate;
etching a pattern in said spacer layer to produce an etched spacer
layer;
forming a silicon oxynitride layer on said etched spacer layer;
and
etching said spacer layer to remove said spacer layer without
removing portions of said oxynitride layer formed on said spacer
layer to form said low stress free-standing oxynitride
microstructure on said substrate.
2. A method according to claim 1, wherein said step of forming said
oxynitride layer is carried out at a temperature of not more than
500.degree. C.
3. A method according to claim 2, wherein said temperature is in
the range of 150.degree. C. to 300.degree. C.
4. A method according to claim 1, wherein said microstructure is in
the form of a bridge.
5. A method according to claim 1, wherein said microstructure is in
the form of a cantilever.
6. A method according to claim 1, wherein said microstructure has a
stress of less than 5.times.10.sup.8 dyne/cm.sup.2.
7. A method according to claim 6, wherein said stress is in the
region of 5.times.10.sup.6 to 1.times.10.sup.8 dyne/cm.sup.2.
8. A method according to claim 1, wherein said spacer layer is
formed of aluminum.
9. A method according to claim 1, wherein said oxynitride layer is
formed by a plasma-enhanced chemical vapor deposition of oxynitride
from a mixture comprising silane, nitrous oxide and nitrogen.
10. A method according to claim 9, wherein said silane, nitrous
oxide and nitrogen are present in a volume ratio of 0.5 to 2
(silane):3 to 12 (nitrous oxide):5 to 20 (nitrogen).
11. A method according to claim 10, wherein said volume ratio of
silane to nitrous oxide to nitrogen is about 1:6:10.
12. A method according to claim 9, wherein said chemical vapor
deposition is carried out at a temperature of about 300.degree. C.
for a time period of about 25 minutes and at a power level of about
45 watts and a pressure of about 300 mtorr.
13. A method according to claim 9, wherein said oxynitride layer
has a thickness of about 1000 to about 25000 Angstroms.
14. A method for forming an integrated silicon sensor comprising a
low stress free-standing oxynitride microstructure, said method
comprising the steps of:
providing a substrate having at least one integrated circuit on the
major surface thereof;
forming a spacer layer on said substrate;
etching a pattern in said spacer layer to produce an etched spacer
layer;
forming a silicon oxynitride layer on said major surface under
conditions which do not adversely affect said integrated circuit;
and
etching said spacer layer to remove said spacer layer without
removing portions of said oxynitride layer formed on said spacer
layer to form said low stress free-standing oxynitride
microstructure on said substrate.
15. A method according to claim 14, wherein said step of forming
said oxynitride layer is carried out at a temperature not higher
than 500.degree. C.
16. A method according to claim 15, wherein said temperature is in
the region of 150.degree. to 300.degree. C.
17. A method according to claim 14, wherein said oxynitride
microstructure comprises a metal layer disposed between two
oxynitride layers.
18. A method according to claim 17, wherein said metal is selected
from the group consisting of aluminum, platinum, nickel, titanium,
tungsten, gold, chromium, silver, palladium, titanium-tungsten,
titanium-platinum, aluminum-silicon, and
aluminum-silicon-copper.
19. A method according to claim 18, wherein said metal is aluminum
and is encapsulated between said layers of oxynitride.
Description
FIELD OF THE INVENTION
The present invention relates to oxynitride frontside
microstructures, and to their fabrication.
BACKGROUND OF THE INVENTION
The application of silicon-based electronics systems, especially
for automotive applications, has seen an almost explosive growth in
the last few years. The silicon-based electronics are used to store
control algorithms, process information, and to direct actuators to
perform various functions, including steering, suspension, and
display of driver information, to name but a few. While the
electronics revolution unfolds, sensor technology, on the other
hand, is not keeping pace, and sensor designs continue to be based
on dated technologies with inbred limitations. Recent trends have
identified silicon as the basis for future sensor technology, and
this hopefully will close this technology gap and permit greater
application of control systems utilizing sensor technology.
Existing control systems use silicon-based electronics, and nearly
all have embedded microprocessors. Silicon is widely recognized in
the industry as being suitable for this application in view of its
high reliability, high strength and low cost. In addition, silicon
sensor designs can be created using a variety of manufacturing
processes, one of the most promising of which is referred to as
"micromachining" which uses chemical processes to introduce
three-dimensional mechanical structures into silicon. These
"microstructures", as they are referred to, can be made sensitive
to specific physical phenomena, such as acceleration, pressure or
fluid flow, by taking advantage of several special properties of
silicon, including piezo resistance, piezo electric and controlled
resistance. For example, a micromachined cantilevered beam produces
a minute resistance change when flexed by the force of
acceleration. However, the output signal from this micromachined
sensor is very small (millivolts), so that additional electronic
circuitry is necessary for signal conditioning and amplification.
These electronic circuits are usually integrated circuit chips
which are interconnected to the micromachined element. Different
aspects of micromachining are reviewed in Lee et al, "Silicon
Micromachining Technology For Automotive Applications", SAE
Publication No. SP 655, February 1986, and the entire content of
that publication is hereby incorporated by reference.
A disadvantage associated with polycrystalline silicon is that it
possesses an inherent high compressive stress. For example, undoped
polycrystalline silicon has a stress of the order of
-5.times.10.sup.9 dyne/cm.sup.2. This high compressive stress is a
disadvantage especially when polysilicon is used for the
fabrication of free-standing microstructures, such as cantilevers
or bridges, which must be mechanically stable and must not buckle
or break. Such structures must have a low level of stress in order
to produce free-standing stable structures of sufficient dimension
to be useful as a sensing element. In a typical polysilicon
deposition process used widely in the fabrication of integrated
circuits today, silane gas is injected into a process tube at low
pressure and a temperature of approximately 625.degree. C. These
processing conditions produce a very uniform layer of deposited
polysilicon material on a substrate. However, the polysilicon layer
and the underlying substrate will produce a net compressive stress
force in the polysilicon and this gives rise to the disadvantages
noted earlier.
Recently, there has been much research into methods for producing
stress-free polycrystalline silicon. These methods have primarily
been to deposit the silicon at a temperature that will produce an
amorphous silicon film having little or no crystalline structure
present. There have been other attempts to anneal the polysilicon
in different ways to relieve the stress. All of the prior methods
suffer from the disadvantage of changing the polysilicon deposition
parameters and utilizing high temperatures (i.e. above 600.degree.
C.) and are incompatible with current technology trends and
processing methods. In particular, the use of high temperatures for
annealing and other processing is precluded if pre-existing
electronic circuitry is present.
SUMMARY OF THE INVENTION
It has now been found that it is possible to fabricate devices such
as microsensors at relatively low temperatures by creating an
oxynitride microstructure on a suitable semiconductor substrate.
Thus, according to one aspect of the present invention; there is
provided a method of producing a microstructure comprising forming
an oxynitride microstructure on the surface of a silicon substrate.
According to another aspect of the present invention, there is
provided a method of forming an integrated silicon sensor
comprising forming an oxynitride microstructure on a major surface
of a substrate having at least one integrated circuit provided on
that major surface, under conditions which do not adversely affect
the integrated circuit. According to a yet further aspect of the
present invention, there is provided a device comprising a
semiconductor substrate and an oxynitride microstructure disposed
on a major surface of the substrate.
It will be appreciated that by fabricating the oxynitride
microstructure at relatively low temperatures, typically not higher
than 500.degree. C., and preferably within the range of about
80.degree. to 450.degree. C., it is possible to fabricate sensors
and other components on a prefabricated integrated circuit without
destroying or otherwise harming the electronics. The method of the
present invention thus facilitates exploitation of the so-called
"foundry" concept in which integrated circuit processing is first
carried out on a silicon wafer and this is followed, at a later
stage, by fabrication of integrated sensor microstructures on
vacant real estate of the wafer. By fabricating the sensor
microstructures at temperatures less than 500.degree. C., and
preferably less than 400.degree. C., it is possible to introduce a
large number of sensors having different architecture without
damaging the electronic circuitry already present on the wafer.
A further advantage realized according to the present invention is
that the fabrication of the microstructures can be controlled so as
to produce a low stress oxynitride. Typically, the stress of the
oxynitride of the microstructures of the present invention is less
than about 5.times.10.sup.8 dynes/cm.sup.2, and may be in the range
of 5.times.10.sup.6 to 5.times.10.sup.8 dynes/cm.sup.2, which
enables the formation of stable and flexible oxynitride bridges and
cantilevers. The material ordinarily used for fabrication of
microstructures is polysilicon but, as indicated earlier,
polysilicon suffers from an inherent compressive stress, and
requires deposition temperatures in excess of 550.degree. C.,
usually in the region of 625.degree. to 650.degree. C. This
inherent compressive stress associated with polysilicon makes that
material somewhat unsuitable for the fabrication of sensors which
rely on a bridge or cantilever-type configuration. The stress of
the oxynitride microstructure can be carefully controlled by
adjusting the ingredients used to form the oxynitride, typically
silane, nitrous oxide and nitrogen.
As a result of the stability and flexibility of the oxynitride
microstructures of the present invention, it is possible to
fabricate free-standing microstructures suitable for use as, for
example, accelerometers, pressure sensors and mass air flow
sensors, such as anemometers. Other sensing functions are also
within the purview of the microstructures of the present invention,
such as, for example, in the automobile industry for detecting fuel
flow rate, valve position and cylinder pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will now be
described with reference to the accompanying drawings, in
which:
FIGS. 1 through 6 show schematically the principal method steps of
the present invention;
FIG. 7 shows a side view of an integrated silicon sensor comprising
an integrated circuit and a cantilever oxynitride
microstructure;
FIG. 8 shows an enlarged cross-sectional view of embodiments of the
cantilever of FIG. 7.
DETAILED DESCRIPTION OF THE INVENTION
It will be understood that, for purposes of simplicity and ease of
description and understanding, the invention will now be described
with respect to the formation of an oxynitride microstructure of an
uncomplicated type. It will be appreciated, however, that in a
typical arrangement, there may be one or more integrated circuit
components and one or more microstructures formed on the same side
of the silicon substrate. In addition, since oxynitride is an
insulator, the sensor comprising oxynitride is adapted for
measurement of the sensed phenomenon, such as fluid flow rate,
pressure or acceleration, utilizing capacitive, piezo electric or
piezo resistive techniques. This is discussed below in connection
with FIGS. 7 and 8.
Referring to FIGS. 1 through 6, there is shown a silicon substrate
2, typically in the form of a wafer, on which there is formed a
spacer layer 4. The spacer layer may be a metal layer such as an
aluminum layer formed by sputtering aluminum at elevated
temperatures, typically about 200.degree. C., using conventional
sputtering technology. Alternatively, the spacer layer 4 may be an
oxide layer, for example a layer of silicon dioxide formed by
oxidizing the silicon substrate 2 at an elevated temperature, for
example 950.degree. to 1100.degree. C., typically 1000.degree. to
1050.degree. C., for a period of about 3 to 6 hours, usually about
4 hours, in the presence of steam. The process in which the silicon
dioxide layer is formed is conventional, and well known to a person
of ordinary skill in this art. The resulting spacer layer 4 is
generally about 1 to 2 microns thick.
The spacer layer 4 is then etched using conventional
photolithography techniques to produce an etched spacer layer. In
this step, the spacer layer is coated with an emulsion 6 of a
standard photoresist material, and subjected to ultraviolet light
through a mask 8 to define a desired pattern in the photoresist
material, as shown in FIG. 2.
The exposed photoresist material is then developed and etched using
conventional techniques to produce an etched spacer layer as shown
in FIG. 3. It can be clearly seen in FIG. 3 that the etched layer 4
has windows 10 extending through to the silicon substrate 2.
An oxynitride layer 12 is then deposited on the etched spacer layer
4 to produce the structure shown in FIG. 4. The oxynitride is
deposited utilizing plasma-enhanced chemical vapor deposition
(PECVD) to produce an oxynitride layer having a thickness of, for
example, between 1000 and 25000 Angstroms, such as 7000 to 8000
Angstroms. The oxynitride layer is formed from a mixture of silane
(silicon tetrahydride), nitrous oxide and nitrogen. The relative
proportions of silane, nitrous oxide and nitrogen are carefully
chosen so as to ensure that the resulting oxynitride layer is of
low stress, i.e. less than 5.times.10.sup.8 dyne/cm.sup.2. It has
been found that this can be achieved by adjusting the relative
amounts of silane, nitrous oxide and nitrogen so that the volume
ratio between those constituents is 0.5 to 2 (silane):3 to 12
(nitrous oxide):5 to 20 (nitrogen), preferably about 1 (silane):6
(nitrous oxide):10 (nitrogen).
The stress of the microstructures of the present invention is
measured by techniques known to persons of ordinary skill in this
art. In particular, the method described by Guckel et al, "A Simple
Technique for the Determination of Mechanical Strain in Thin Films
with Application to Polysilicon", J. App. Phys., 1671, 1985 may be
used to measure the strain in the silicidated microstrucure. The
stress is then calculated from a knowledge of known mathematical
techniques. An alternative method for measuring the stress is to
use a stress guage, such as the one manufactured by Ionic Systems
Inc. under the model number 30122. Such a guage measures the
average stress across the wafer.
The deposition of the oxynitride layer is carried out at a
temperature of not more than 500.degree. C., and is preferably in
the region of 150.degree. to 300.degree. C. The deposition is
effected under reduced pressure, typically in the region of about
200 to 400 microtorr (mtorr), preferably about 300 mtorr.
Ordinarily, the deposition is carried for a period of about 20 to
40 minutes, depending on the desired thickness of oxynitride layer,
and at a power level of about 40 to 60 watts.
It has been found, according to a preferred embodiment, that an
oxynitride layer having a thickness of about 7200 Angstroms can be
obtained by depositing oxynitride under conditions of
plasma-enhanced chemical vapor deposition using silane, nitrous
oxide and nitrogen in a volume ratio of about 1:6:10 at a pressure
of about 300 mtorr, a temperature of about 300.degree. C., over a
time period of about 20 minutes at a power level of 45 watts.
Following deposition, the oxynitride layer 12 is then subjected to
etching using conventional photoresist techniques. This produces an
etched oxynitride layer 12 as shown in FIG. 5.
FIG. 6 shows the result of etching the spacer layer (or sacrificial
layer) 4 to give a low stress free-standing microstructure 14. As
will be seen from FIG. 6, the microstructure 14 can possess
cantilever portions 16 or bridge portions 18 which are stable and
do not buckle or break in view of the absence of tensile or
compressive stress in the oxynitride material. As noted earlier,
the stress of the oxynitride layer is less than 5.times.10.sup.8
dyne/cm.sup.2, and preferably less than 1.times.10.sup.8
dyne/cm.sup.2.
A particularly preferred aspect of the present invention is
illustrated in FIG. 7. In that Figure, there is shown an oxynitride
microstructure 20 formed on the frontside 22 of the silicon
substrate 2 in close proximity to the integrated circuit 24. The
fabrication of such frontside microstructures is made possible by
the fact that the present invention is carried out at temperatures
not higher than 500.degree. C., and preferably less than
400.degree. C. so that adjacent integrated circuit electronics are
not subjected to heat damage. A further important advantage
associated with this approach is that all of the processing and
manipulation of the wafer is effected on one side of the silicon
substrate (i.e. the frontside), thereby obviating the need to
effect processing manipulation on both sides of the wafer, such as
is required when using conventional back-etch techniques. The
overall strength of the integrated sensor is thereby increased, and
the overall cost of production is reduced.
FIG. 8 shows a cantilever of the invention adapted for measuring
acceleration as reflected by flexing of the cantilever terminal
portion 26. In the embodiment shown in FIG. 8, the cantilever 20
has a metal layer 30 sandwiched between two layers of oxynitride
32, 34. Such a structure may be fabricated using conventional
deposition techniques, e.g. sputtering or evaporation, discussed
earlier. The metal may be selected from aluminum, platinum, nickel,
titanium, tungsten, gold, chromium, silver, palladium,
titanium-tungsten, titanium-platinum, aluminum-silicon,
aluminum-silicon-copper. The preferred metal layer is aluminum. The
layer can be present as a thin layer, for example not more than
1000 Angstroms thick. While the preferred structure shown in FIG. 8
contains three layers, it is possible to use two layers or more
than three layers. Whichever arrangement is employed, it is
important to encapsulate the metal layer (as shown in FIG. 8),
especially when the layer is aluminum, to minimise corrosion and
wear.
In the FIG. 8 embodiment, the capacitive change is being measured
as a result of flexing of the cantilever 20 with respect to the
substrate 2. Alternatively, however, it is possible to measure the
movement of the cantilever by use of a piezoelectric or
piezoresistive element such as that shown at 36. The element is
disposed on a highly stressed part of the cantilever structure 20
and detects movement of the free end of the cantilever. Any
suitable piezoelectric material, for example zinc oxide, or
piezoresistive material, for example silicon, may be used.
The invention will now be further described with reference to the
following Example.
EXAMPLE
Three silicon wafers having an aluminum film formed on the surface
thereof were prepared using conventional electron (E)-beam
techniques at 200.degree. C. The thickness of the aluminum film in
each instance was about 2 microns. An emulsion of standard
photoresist material was then applied to the aluminum film of each
of the three wafers, and each were exposed to ultraviolet light
through a standard contact mask. Each wafer was then developed and
subjected to etching using standard procedures to etch the aluminum
film down to the silicon in accordance with the pattern of the
mask.
The three wafers thus formed were then treated as follows:
Sample 1
A nitride film was deposited on the wafer using plasma-enhanced
chemical vapor deposition under the following conditions:
SiH.sub.4 --19% (11 sccm)
NH.sub.3 --10% (7.2 sccm)
N.sub.2 --18% (189 sccm)
The PECVD was carried out at 300.degree. C. for 35 minutes at a
power level of 57 watts and a pressure of 400 mtorr. This resulted
in a nitride layer having a thickness of about 8000 Angstroms.
Sample 2
A nitride layer was deposited on the wafer using PECVD under the
following conditions:
SiH.sub.4 --17% (8.5 sccm)
NH.sub.3 --55% (41.25 sccm)
N.sub.2 --10% (100 sccm)
The PECVD was carried out at 350.degree. C. for 34 minutes at a
power level of 57 watts and a pressure of 350 mtorr. This resulted
in a nitride layer having a thickness of about 7300 Angstroms.
Sample 3
An oxynitride layer was deposited on the wafer under the following
conditions:
SiH.sub.4 --20% (10 sccm)
N.sub.2 O--80% (60.0 sccm)
N.sub.2 --10% (100 sccm)
The PECVD was carried out at a temperature of 300.degree. C. for 23
minutes at a power level of 45 watts and a pressure of 300 mtorr.
This resulted in a oxynitride layer having a thickness of about
7200 Angstroms.
Each wafer was then subjected to photolithography using
conventional techniques followed by reactive ion etching (RIE) with
a nitride etch, using a power level of 60 watts (20%) under a
pressure of 90 mtorr with a CF.sub.4 /O.sub.2 mixture introduced at
a flow rate of 16 sccm (40%). The etch time was about 10 to 20
minutes.
Finally, a sacrificial layer etch was carried out using potassium
hydroxide or "pirahana" (a hydrogen peroxide/sulphuric acid
mixture) to remove remaining aluminum and produce a free-standing
microstructure.
Samples 1 and 2 (with the nitride layers) collapsed due to high
stress present in the nitride layer. Sample 3, on the other hand,
resulted in a stable, low stress oxynitride free-standing structure
which did not collapse or buckle, as evidenced by scanning electron
microscope (SEM) photography.
* * * * *